This invention relates to adsorption processes, and more particularly to hydrogen production via pressure swing adsorption (PSA) and vacuum swing adsorption processes.
Hydrogen production via pressure swing adsorption (H.sub.2 PSA) is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metals refining and other related industries. Typical commercial sources for the production of hydrogen are the reforming of natural gas or partial oxidation of various hydrocarbons. Other hydrogen-rich gas sources which can be upgraded by PSA technology to a high purity product include refinery off-gases with C.sub.1 -C.sub.10 hydrocarbon contaminants. See, e.g., U.S. Pat. No. 3,176,444 to Kiyonaga. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g., air or oxygen-enriched air), producing a hydrogen gas stream containing accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless it is desired to recover carbon monoxide, the carbon monoxide is customarily converted to carbon dioxide by water gas shift reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then sent to a PSA system.
In a typical PSA system, a multicomponent gas is passed to at least one of multiple adsorption beds at an elevated pressure to adsorb at least one strongly sorbed component while at least one component passes through. In the case of H.sub.2 PSA, H.sub.2 is the most weakly adsorbed component which passes through the bed. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more concurrent steps which permits essentially pure H.sub.2 product to exit the bed with a high recovery of the most weakly adsorbed component, H.sub.2. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization.
The cost of hydrogen from integrated reformer/PSA systems is impacted by both the capital and operating costs of the system. Clearly, economic production of hydrogen requires minimization of operating and capital costs. Capital cost is most widely affected by the size of the reformer and the size of the PSA beds. PSA bed size decreases as the feed loading (lb-moles of feed gas processed/bed volume) of the PSA increases. Feed loading can be increased by either improved process cycles or improved adsorbents. The size of the reformer is impacted mostly by the hydrogen recovery of the PSA. Improvements in hydrogen recovery in the PSA result in smaller reformer size (the reformer does not need to produce as much hydrogen because of better recovery in the PSA). Improvements in hydrogen recovery also lead to a reduced demand for reformer feed gas, i.e., natural gas, which constitutes the largest operating cost of the reformer. Hydrogen recovery in the PSA can also be improved by either improved process cycles or improved adsorbents.
H.sub.2 PSA process performance (on-line time, feed loading, product purity, recovery) is usually dictated by the second most weakly adsorbing component in the H.sub.2 -rich stream. A bed can stay on feed, producing pure H.sub.2, only until the level of impurity breakthrough reaches the desired product purity. For steam/methane reformer (SMR) cases, the PSA feed gas composition is typically about 1% N.sub.2, 5% CH.sub.4, 5% CO, 18% CO.sub.2 and the remainder H.sub.2. To produce high purity H.sub.2 (99.99+%) with this feed gas composition, N.sub.2 is the key breakthrough component since it is the most weakly adsorbing feed gas component besides H.sub.2. Since N.sub.2 is the key breakthrough component, it has been common to place a zeolite adsorbent with high capacity for N.sub.2 at the product end of the bed. In some cases, the H.sub.2 purity spec is 99.9% with less than 10 ppm CO in the product H.sub.2. In these cases, the plant becomes CO-controlling and zeolites are the prior art adsorbents for CO removal from H.sub.2.
For example, U.S. Pat. No. 3,430,418 to Wagner teaches a layered adsorption zone with the inlet material comprising activated carbon and the discharge end containing zeolite for the removing the minor component of N.sub.2, CO or CH.sub.4. U.S. Pat. No. 3,564,816 to Batta exemplifies the use of CaA (5A) zeolite as an adsorbent for PSA processing. U.S. Pat. No. 3,986,849 to Fuderer et al. discloses a layered bed adsorption zone with activated carbon at the feed end of the bed and CaA zeolite at the discharge end.
The art teaches a variety of means for removing CO and/or N.sub.2 from gas mixtures. In particular, Li containing X and Ca containing A type zeolites have been widely employed as adsorbents for separating N.sub.2 or CO from more weakly adsorbing gas mixtures. See, e.g., U.S. Pat. Nos. 4,813,980, 4,859,217, 5,152,813, 5,174,979, 5,354,360 and 5,441,557, 5,912,422, EP 0 855 209 and WO 97/45363.
Despite the foregoing developments and their asserted advantages, there is still room for improvement in the art.
Thus, it is desired to provide an improved method for recovering purified hydrogen in CO and/or N.sub.2 controlled H.sub.2 PSA. It is also desired to provide improved adsorbents and systems for use in the improved method.
It is further desired to provide an improved CO coldbox offgas purification method. It is also desired to provide improved adsorbents and systems for use in the improved method.
All references cited herein are incorporated herein by reference in their entireties.